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Ionophores are a specific type of antibiotic that are sometimes added to cattle and chicken feed to promote growth and to prevent coccidial infections. In cattle, ionophores select for a more beneficial rumen microflora and improve feed efficiency.

Ionophores have a pharmacologic effect by changing the flux of certain electrolytes across cell membranes. The doses of ionophores normally used in feed for other animals can be highly toxic to horses. Cardiac muscle, skeletal muscle, and the neurologic system are affected by ionophore toxicity. Changes in movement of ions across cell membranes can kill cells, and muscle cells are very susceptible to the toxic effects of ionophores. The most commonly used ionophores are monensin, lasalocid, and salinomycin.

Clinical signs of ionophore toxicity in horses often include sudden death. Other clinical signs are colic, weakness, unsteady gait, diarrhea, and excessive urination. Blood work will usually reveal elevated muscle enzymes. Animals that are acutely poisoned may show no clinical signs other than sudden death.

There is no specific treatment for ionophore toxicity. Any feed that is suspected of being contaminated should be collected immediately and no longer fed. If horses are known to have recently ingested ionophores, treatment to flush stomach contents and minimize absorption is indicated. Supportive care such as intravenous fluids and pain relief is given to surviving animals. Vitamin E and selenium may help to offset muscle damage. The overall prognosis for ionophore toxicity in horses is poor to grave.

Horses that survive ionophore toxicity may have permanently impaired heart function. Electrocardiograms (measures the electrical activity of the heart) and echocardiograms (ultrasound of the heart) can be helpful to assess heart function and determine if horses can be put back into work.

It is essential that any cattle or chicken feed containing ionophores be stored away from horse feed and not fed where horses can have any access to it. Feed manufacturers are vigilant to ensure that there is no cross-contamination when horse feed is produced in the same facility as ionophore-containing feeds. Suspect feed can be tested for the presence and level of ionophore contamination by several different laboratories.

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